1. Field of the Invention
The present invention relates to dewatering blades in a dewatering instrument applicable to a twin-wire former of a paper making machine.
2. Description of the Prior Art
Generally in a twin-wire former, stock located between the two wires is dewatered during its travel by various dewatering instruments and thereby gradually assumes the form of a fiber mat. Thus, a paper web is formed.
The general structure of a twin-wire former in the prior art is shown in FIG. 6, and details of a representative fixed dewatering instrument employed by such a twin-wire former are shown in FIG. 5. In FIGS. 5 and 6, stock 9 ejected from a headbox 5 is dewatered simultaneously to both the upper and lower sides of the wires, due to a squeezing effect created by the tension of the wires, in a gap 10 just behind a breast roll 3 and a forming roll 4. The stock is also dewatered by a pulsating pressure acting upon the stock in the downstream fixed dewatering equipment 6. Water extracted upward at that time is scraped out by an auto-slice blade 7a and is accumulated in an auto-slice 7 to be exhausted.
FIG. 4 shows prior art dewatering blades of a fixed dewatering instrument and a dewatering pressure profile generated in the fixed dewatering instrument. Dewatering blades 6b are supported from T-bars 6c disposed along a circular arc, having a radius of curvature R.sub.2 ', on a main body of the dewatering equipment and are extractible in the widthwise direction of the equipment. Accordingly, the top surfaces of the dewatering blades 6b lie on a circular arc having an approximate radius of curvature R.sub.2. However, because of the fact that the top surface of each dewatering blade 6b is planar and spaces are present between the blades, the actual path along which the wire travels has the shape of a sector of a polygon. A pulse-shaped pressure generated in the dewatering blade section is considered to be caused by variations of a moment applied to the stock when the stock between two wires travels beyond the blades. In addition, as wrap angles (inlet side: .theta..sub.1, outlet side: .theta..sub.2) of the wires at blade end portions, which angles are a predetermined factor in the generation of pressure, become large, the generated pressure also becomes large. The wrap angle is determined by the radius of curvature R.sub.2 and a spacing L or L' between the blades, and the smaller the radius of curvature is, and the larger the spacing is, the larger the wrap angle becomes.
In FIG. 4, since the radius of curvature R.sub.2 has a predetermined value, as the spacing L becomes larger, the wrap angle also becomes larger. Consequently, a larger pulsating pressure can be obtained. (Solid lines indicate the case where the blades are disposed only on every other T-bar, and solid lines and dashed lines jointly depict the case where a full number of blades are used.)
The auto-slide blade 7a is normally set at such a location that it will contact a second wire 2 under the condition where stock is not present. However, during operation, because of the thickness of the stock, the wire has wrap angles at the rear end of the final dewatering blade and at the front end of the auto-slice blade 7a. Therefore, pressure similar to that in the fixed dewatering equipment will be generated between these blades.
While it has been generally known that a shearing force acts on the stock due to the pressure pulses generated in the fixed dewatering instrument, whereby a dispersion of fibers is promoted, if a large pressure is applied to the stock from the time of initial mat formation when the fibers are more free to move, then a well-formed mat is produced. On the other hand, fibers in a middle portion of the mat become oriented to a high degree. In addition, since the dewatering blades form the outer layers of the mat adjacent the first wire and the second wire differently, if a strong squeezing action is applied to the stock at the time of initial mat formation (i.e. if a large inlet side wrap angle is used), then the porosity of the paper layer adjacent the first wire becomes high, a yield of micro-fine fibers and an ash component become poor, and the difference in characteristics between the respective outer layers, namely the sides of the paper (micro-fine fiber distribution, ash component distribution, ink-absorbing property, etc.) becomes great. In other words, it becomes difficult to obtain a uniform mat (see Japanese Pat. Appln. No. 2-199230 (1990) [Laid-Open Japanese Patent Specification No. 4-91287 (1992)]).
Accordingly, the pulsating pressure applied to the stock should be controlled depending upon the degree of formation of the mat. However, in the prior art, such a control must be effected by changing the blades because the radius of curvature R.sub.2 is fixed. However, the changing of the blades requires stopping the paper making machine and hence is inefficient and is not practiced very much in the prior art.
Moreover, if the thickness of stock passing through the auto-slice blade section is too thick, the wire wrap angle becomes excessive and, if a gap is provided between the blade 7a and the second wire 2 in order to avoid this, then the upwardly extracted water easily passes through the gap between the blade 7a and the second wire. In either case, there is a possibility of destroying the formed mat. As described above, the positioning of the auto-slice 7 is a delicate operation. Therefore, changing the position of the auto-slice 7 according to the prevailing paper making conditions has not been practiced.